U.S. patent number 5,212,819 [Application Number 07/683,419] was granted by the patent office on 1993-05-18 for liquid crystal display device with particular angle between optical axis of optically anisotropic material and observation direction.
This patent grant is currently assigned to Seiko Epson Corporation. Invention is credited to Hiroshi Wada.
United States Patent |
5,212,819 |
Wada |
May 18, 1993 |
Liquid crystal display device with particular angle between optical
axis of optically anisotropic material and observation
direction
Abstract
A super twisted nematic liquid crystal display device includes a
liquid crystal cell, upper and lower polarizing plates, and at
least one phase difference plate between the polarizing plates. The
angle between the direction of the optical axis of the phase
difference plate and the observation direction of the device is set
between about 0.degree.-30.degree. or 60.degree.-90.degree..
Inventors: |
Wada; Hiroshi (Suwa,
JP) |
Assignee: |
Seiko Epson Corporation (Tokyo,
JP)
|
Family
ID: |
14088751 |
Appl.
No.: |
07/683,419 |
Filed: |
April 9, 1991 |
Foreign Application Priority Data
Current U.S.
Class: |
349/102; 349/117;
349/119; 349/121; 349/181 |
Current CPC
Class: |
G02F
1/13363 (20130101); G02F 1/1396 (20130101); G02F
2413/08 (20130101); G02F 1/133531 (20210101); G02F
2202/40 (20130101); G02F 2413/02 (20130101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/139 (20060101); G02F
1/1335 (20060101); G02F 001/13 (); G02F
001/1335 () |
Field of
Search: |
;350/347R,337
;359/63,73,102 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
297841 |
|
Apr 1989 |
|
EP |
|
0350063 |
|
Jan 1990 |
|
EP |
|
0350075 |
|
Jan 1990 |
|
EP |
|
0352724 |
|
Jan 1990 |
|
EP |
|
2580105 |
|
Oct 1986 |
|
FR |
|
123944 |
|
Oct 1978 |
|
JP |
|
Other References
Ohgawara et al., "A Color STN Display with Two Retardation
Compensating Films", 1989 SID International Symposium, Digest of
Technical Papers, First Edition, May, 1989, pp. 390-392..
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Gross; Anita Pellman
Attorney, Agent or Firm: Blum; Kaplan
Claims
What is claimed is:
1. A liquid crystal display device, comprising:
a twisted nematic liquid crystal display cell including a pair of
spaced apart opposed substrates with electrodes selectively
disposed thereon and a twisted nematic liquid crystal material
disposed in the space between the substrates;
a pair of polarizing plates disposed on opposite sides of the
liquid crystal cell, one of which defines a display screen; and
at least one layer of an optically anisotropic substance disposed
between the liquid crystal cell and one of the pair of polarizing
plates;
the display screen having a rear edge as viewed from a user and an
observation direction which is both in the plane of the display
screen and substantially perpendicular to the rear edge thereof,
the observation direction being the direction of the projection in
the plane of the display screen of the direction from which a user
of said display device most frequently views said display screen,
the device being constructed and arranged to have a viewing angle
with respect to the observation direction at which the display will
have acceptable contrast to accurately convey information to a
user, the angle between the direction of the optical axis of the
optically anisotropic substance layer and the observation direction
of the display device being set at a predetermined angle to produce
substantially bilateral symmetry of the contrast distribution with
respect to the observation direction as the user moves in the
direction of the viewing angle .beta. between a projection of the
direction of viewing by the user in the plane of the display screen
and the observation direction or in the direction of the viewing
angle .alpha. between the direction of viewing of the user and an
axis perpendicular to the observation direction and the plane of
the display screen.
2. The liquid crystal display device of claim 1, wherein the liquid
crystal material is twisted at least about 120.degree..
3. The liquid crystal display device of claim 1, wherein the
predetermined angle is between one of about 0.degree. and
30.degree. and about 60.degree. and 90.degree..
4. The liquid crystal display device of claim 3, wherein the
optically anisotropic substance layer is made of a uniaxial high
molecular weight polymers sheet.
5. The liquid crystal display device of claim 4, wherein the
polymer sheet is a polymer selected from the group consisting of
polycarbonate, diacetyl cellulose, polyamide, polyimide, polyether
sulfone, polysulfone, polyolefin, polyethylene, polyethylene
terephthalate, polyvinyl alcohol, acryl, and polymethyl
methacrylate.
6. The liquid crystal display device of claim 5, wherein the
polymer is polycarbonate.
7. The liquid crystal display device of claim 1, wherein the device
includes at least two optically anisotropic layers of uniaxial high
molecular weight polymer sheets disposed between the liquid crystal
cell and one of the pair of polarizing plates, and wherein the
predetermined angle between the direction of the optical axis of at
least one of the sheets and the display device direction of
observation is between one of about 0.degree. and 30.degree. and
about 60.degree. and 90.degree..
8. The liquid crystal display device of claim 1, wherein the device
includes at least one optically anisotropic substance layer between
the liquid crystal cell and one of the pair of polarizing plates
and at least another optically anisotropic substance layer between
the liquid crystal cell and the other polarizing plate, and the
angle between the direction of the optical axis of at least one of
the optically anisotropic substance layers and the display device
direction of observation is between one of about 0.degree. and
30.degree. and about 60.degree. and 90.degree..
9. The liquid crystal display device of claim 8, wherein the
optically anisotropic substance layers are a uniaxial high
molecular weight polymer.
10. The liquid crystal display device of claim 1, wherein the
device includes at least one optically anisotropic substance layer
between the liquid crystal cell and one of the pair of polarizing
plates and at least another optically anisotropic substance layer
between the liquid crystal cell and the other polarizing plate, and
the angle between the bisector of the optical axes of the two
optically anisotropic sheets adjacent to the liquid crystal cell
and the display device direction of observation is between one of
about 0.degree. and 30.degree. and about 60.degree. and
90.degree..
11. The liquid crystal display device of claim 10, wherein each of
the optically anisotropic substrate layers is a uniaxial high
molecular weight polymer layer.
12. A liquid crystal display device having a display screen with a
rear edge as viewed from a user, comprising:
a super twisted nematic liquid crystal cell including first and
second opposed electrode substrates with a twisted nematic liquid
crystal material therebetween with an angle of twist between about
180.degree. and 260.degree. from the first substrate to the second,
the cell having an observation direction, the observation direction
being a direction of the projection in the plane of the display
screen of the direction from which a user of said display device
most frequently views said display screen, the observation
direction being both in the plane of the display screen and
substantially perpendicular to the rear edge thereof;
a pair of polarizing plates disposed on opposite sides of the
liquid crystal cell with .THETA..sub.1 being the angle between the
observation direction and the axis of polarization of the first
polarizer adjacent the first substrate of the liquid crystal cell
and .THETA..sub.2 being the angle between the observation direction
and the axis of polarization of the other polarizer; and
at least one layer of an optically anisotropic material between the
liquid crystal cell and one polarizer with .THETA..sub.3 being the
angle between the optical axis of the at least one layer of
optically anisotropic material and the observation direction,
.THETA..sub.3 between one of about 0.degree. and 30.degree. and
about 60.degree. and 90.degree..
13. The liquid crystal display device of claim 12, wherein
.THETA..sub.1 is between about 15.degree. and 75.degree..
14. The liquid crystal display device of claim 13, wherein
.THETA..sub.2 is between about 0.degree. and 45.degree..
15. The liquid display device of claim 13, wherein the first
substrate has a rubbing direction, and an angle .THETA..sub.4 is
defined between the observation direction and the rubbing direction
of the first substrate and is between about 55.degree. and
90.degree..
Description
BACKGROUND OF THE INVENTION
This invention relates generally to a liquid crystal display
device, and more particularly, to a liquid crystal display device
having improved viewing angle and improved black and white display
contrast.
Super twisted nematic (STN) liquid crystal display devices
including a uniaxial anisotropic member (an oriented high molecular
weight polymer sheet) between the polarizers of the device have
been proposed to improve the display contrast. An example of such a
liquid crystal display device 10 with a uniaxial optically
anisotropic member 12 is shown in FIG. 1. Device 10 includes a
twisted nematic liquid crystal display cell 13 with an upper linear
polarizer 11 and a lower polarizer 14 on the outer surfaces of cell
13 and optically anisotropic layer 12. Display cell 13 includes an
upper substrate 15 and lower substrate 16 with transparent
electrodes 17 and 18 disposed on the inner surfaces and a twisted
nematic liquid crystal material 19 therebetween. A spacer 22 holds
substrates 15 and 16 apart and liquid crystal material 19
therebetween. Liquid crystal material 19 is twist oriented by
rubbing the interior surfaces of substrates 15 and 16.
FIG. 2 shows the relationship between the axes of these elements.
In FIG. 2, R15 and R16 designate the rubbing directions of upper
substrate 15 and lower substrate 16. Angle T4 designates the
direction and angle of twist of the liquid crystal molecules in
material 19 from upper substrate 15 to lower substrate 16. P11 and
P12 designate the directions of the axes of polarization of upper
polarizer 11 and lower polarizer 12, respectfully. A line A--A'
identifies the direction of observation of device 10.
.THETA..sub.45 is the angle between direction of observation A--A'
and rubbing direction R15 of upper substrate 15 and .THETA..sub.46
is the angle between direction of observation A--A' and rubbing
direction R16 of lower substrate 16. .THETA..sub.45 and
.THETA..sub.46 are approximately equal to each other. Device 10 is
described in detail in applicant's U.S. Pat. No. 4,844,569, the
contents of which are incorporated herein by reference.
In device 10, the retardation value of optically anisotropic member
12 and the relationship between the axes are set for use in an STN
display mode in order to improve the contrast and allow for a
black-and-white display. While this arrangement has improved
contrast and achieved nearly a full black and white display
compared to ordinary STN devices, viewing angle characteristics
were not considered.
Accordingly, it is desirable to provide a liquid crystal display
device which has improved viewing angle characteristics as well as
excellent black and white hues in the display.
SUMMARY OF THE INVENTION
Generally speaking, in accordance with the invention, a liquid
crystal display device includes a twisted nematic liquid crystal
display cell and at least one optically anisotropic member disposed
between a pair of polarizing plates disposed on opposite sides
thereof and a twisted nematic liquid crystal material disposed in
the liquid crystal cell. The direction of the optical axis of the
optically anisotropic substance layer is at a predetermined angle
from the usual direction of observation of the display device.
Preferably, this angle is between about 0.degree. and 30.degree. or
between about 60.degree. and 90.degree..
Accordingly, it is an object of the invention to provide an
improved liquid crystal display device having improved view angle
characteristics.
Another object of the invention to provide a liquid crystal display
device with an anisotropic layer between the polarizers of the
device.
A further object of the invention is to provide a liquid crystal
display device having true shades of black and white in the
display.
Yet another object of the invention is to provide a super twisted
liquid crystal display device having improved viewing angle
characteristics.
Still other objects and advantages of the invention will, in part,
be obvious and will, in part, be apparent from the
specification.
The invention accordingly comprises the article of manufacture
possessing the features, properties and the relation of elements
which will be exemplified in the article hereinafter described, and
the scope of the invention will be indicated in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the invention, reference is had to
the following description taken in connection with the accompanying
drawings, in which:
FIG. 1 is a cross-sectional view of a liquid crystal display device
constructed and arranged in accordance with the prior art;
FIG. 2 diagrammatically illustrates the axes of the elements of the
device of FIG. 1;
FIG. 3 diagrammatically illustrates the normal direction of
observation;
FIG. 4 is a cross-sectional view of a liquid crystal display device
constructed and arranged in accordance with a first embodiment of
the invention;
FIG. 5 diagrammatically illustrates the axes of the elements of the
device of FIG. 4;
FIG. 6 diagrammatically illustrates equal contrast curves for the
device of FIG. 3;
FIG. 7 is a cross-sectional view of a liquid crystal display device
construed and arranged in accordance with a second embodiment of
the invention;
FIG. 8 diagrammatically illustrates the axes of the elements of the
device of FIG. 7;
FIG. 9 is a cross-sectional view of a liquid crystal display device
constructed and arranged in accordance with a third embodiment of
the invention; and
FIG. 10 diagrammatically illustrates the axes of the elements of
the device of FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As used herein, the direction of observation is defined with
reference to FIG. 3. The direction of observation is the projection
in the plane of the display screen 53 of the direction from which
the display screen is viewed most frequently during use of the
liquid crystal display 40. Ordinarily, it coincides with the
direction A--A' perpendicular to the lengthwise direction and rear
edge 54 of the rectangular display 40. In other words, the
observation direction as used herein is the direction in the plane
of the liquid crystal screen 53 that extends from the user to the
device 40 along which a projection in said plane of the user's most
frequent line of sight would lie. As used herein and as shown in
FIG. 3, .alpha. designates the viewing angle between the direction
OC perpendicular to the plane of the display screen and the
direction of viewing OD, and .beta. designates the projected
viewing angle between the direction of observation and the
direction projected of viewing OD' on the plane of display screen
53.
It was desired to achieve display viewing angle characteristics
such that the viewing angles on the front left side and front right
side with respect to the direction of observation are approximately
equal to each other, and that the ranges of viewing angles are
generally uniform in the horizontal direction. An examination was
made as to the effect on viewing angle characteristics of the
directions of the polarization axes of the polarizing plates, the
directions of axes including the rubbing directions of the liquid
crystal cell, and the angles between the axes. This examination was
made by using equal-contrast curves (the relationship between
values of angles .alpha. and .beta. shown in FIG. 3 at which a
certain contrast is obtained). It was found that the direction of
the center line on which equal-contrast curves are generally
symmetric approximately coincides with the direction of the optical
axis of the optically anisotropic member.
To achieve improved viewing angle characteristics in the liquid
crystal display device in accordance with the invention, the angle
between the observation direction A--A' and the direction of the
optical axis of the phase difference plate is preferably between
about 0.degree.-30.degree. or 60.degree.-90.degree..
A liquid crystal display device 40 constructed and arranged in
accordance with the invention is shown in FIG. 4. Device 40
includes a liquid crystal display cell 43 and a phase difference
plate 42 disposed between an upper linear polarizer 41 and a lower
polarizer 44. Phase difference plate 42 is a uniaxially-oriented
polycarbonate member used as an optically anisotropic layer.
Phase difference plate 42 need not be limited to
uniaxially-oriented polycarbonate. For instance, phase difference
plate 42 may be formed by uniaxially stretching a material selected
from, but not limited to, monomers or polymers of diacetyl
cellulose, polyamide, polyimide, polyether sulfone, polysulfone,
polyolefin, polyethylene, polyethylene terephthalate, polyvinyl
alcohol, acryl, and polymethyl methacrylate.
FIG. 4 schematically shows a cross-sectional view of the structure
of twisted nematic liquid crystal device 40 having a single layer
42 of an optically anisotropic substance. A liquid crystal cell 43
and layer 42 are disposed between an upper polarizer 41 and a lower
polarizer 44. Cell 43 includes a twisted nematic liquid crystal
material 49 disposed between an upper substrate 45 and lower
substrate 46 with transparent electrodes 47 and 48 disposed on the
interior surfaces, respectively.
Nematic liquid crystal material 49 is twist-oriented by rubbing the
interior surfaces of substrates 45 and 46. Although liquid crystal
material 49 can be oriented other than by rubbing, for convenience
of description the direction in which the major axes of liquid
crystal material 49 adjacent to substrates 45 and 46 will
hereinafter be referred to as "rubbing direction". Spacer 52 serves
to hold substrates 45 and 46 apart and retains liquid crystal
material 49 in cell 43. Spacing maintaining members such as glass
fibers or glass balls may be dispersed between substrates 45 and 46
to maintain the thickness of liquid crystal material 49.
The relationship between the axes of polarizers 41 and 44, phase
difference plate 42 and cell 43 are shown in FIG. 5. The rubbing
directions of upper substrate 45, lower substrate 46 and phase
difference plate 42 are denoted by R45, R46 and R42, respectively.
The polarizing axes (absorption axes) of linear polarizers 41 and
44 are denoted by P41 and P44, respectively. Line A--A' represents
the normal or usual direction of observation of device 40.
The direction and angle of twisting of liquid crystal material 49
in liquid crystal cell 43 as viewed in FIG. 4 from above to below
is designated as T. .THETA..sub.1 is the angle between observation
direction A--A' and direction P41 of the polarization axis of upper
polarizing plate 41; .THETA..sub.2 is the angle between the
observation direction A--A' and direction P44 of the polarization
axis of lower polarizing plate 44; .THETA..sub.3 is the angle
between the observation direction A--A' and the direction R42 of
the optical axis of the phase difference plate 42; and
.THETA..sub.4 is the angle between the observation direction A--A'
and the rubbing direction R45 of upper substrate 45. The values of
angles .THETA..sub.1 through .THETA..sub.4 are positive when
measured clockwise from the observation direction A--A'.
This embodiment illustrated in FIGS. 4 and 5 will be described with
respect to a negative display mode and the orientation of the
polarization axes illustrated. In this mode the screen is dark when
no voltage is applied and bright when a voltage is applied.
However, it is observed that the same benefits of the invention can
be obtained for other orientations as in a positive display mode.
In this positive display mode the screen is bright when no voltage
is applied, or dark when a voltage is applied. The refractive index
anisotropy .DELTA.n of phase difference plate 42 is defined as
.DELTA.nf, and the thickness of plate 42 is defined as df.
When a structure as shown in FIG. 4 has the product
.DELTA.n.multidot.d of the refractive index anisotropy .DELTA.n of
liquid crystal material 49 and a liquid crystal layer thickness of
0.9 .mu.m, liquid crystal 49 is twisted by angle T of 240.degree.;
the product .DELTA.nf.multidot.df of the refractive index
anisotropy .DELTA.nf and the thickness df of phase difference plate
42 is 0.55 .mu.m, .THETA..sub.1 =-45.degree., .THETA..sub.3
=-15.degree., .THETA..sub.3 =0.degree. and .THETA..sub.4
=90.degree.. Equal-contrast curves such as those shown in FIG. 6
are obtained. These curves are symmetric with respect to normal
observation direction A--A' of device 40 and device 40 is easy to
view under these conditions.
The parameters defined above are set forth in Table 1, Examples
1-10 and Comparative Examples 1-3. The resulting display contrast,
dependent on whether the contrast curves were generally symmetrical
with respect to the direction of observation, is also set forth.
The symbols .circleincircle., .largecircle., and x are used to
indicate whether the display is very easy to see
(.circleincircle.), good (.largecircle.), or poor (x).
TABLE 1
__________________________________________________________________________
.DELTA.nf .multidot. df .DELTA.n .multidot. d of phase Twisting of
liquid difference Contrast Angle T crystal plate .THETA..sub.1
.THETA..sub.2 .THETA..sub.3 .THETA..sub.4 symmetry
__________________________________________________________________________
Examples of the invention 1 240.degree. 0.90 .mu.m 0.55 .mu.m
-45.degree. -15.degree. 0.degree. 90.degree. .circleincircle. left
2 .uparw. .uparw. .uparw. -30.degree. 0.degree. 15.degree.
-75.degree. .circleincircle. 3 .uparw. .uparw. .uparw. -15.degree.
15.degree. 30.degree. -60.degree. .largecircle. 4 .uparw. .uparw.
.uparw. -75.degree. -45.degree. -30.degree. 60.degree.
.largecircle. 5 .uparw. 0.85 .mu.m 0.60 .mu.m -45.degree.
-15.degree. 0.degree. 80.degree. .circleincircle. 6 .uparw. .uparw.
.uparw. -40.degree. -15.degree. 10.degree. 90.degree.
.circleincircle. 7 .uparw. 0.80 .mu.m 0.55 .mu.m -45.degree.
-15.degree. 0.degree. 90.degree. .circleincircle. 8 180.degree.
0.85 .mu.m 0.55 .mu.m -45.degree. -40.degree. 0.degree. 90.degree.
.circleincircle. left 9 230.degree. 0.85 .mu.m 0.55 .mu.m
-50.degree. -5.degree. 0.degree. 75.degree. .circleincircle. left
10 .uparw. .uparw. .uparw. -70.degree. -25.degree. -20.degree.
55.degree. .largecircle. Comparative examples 1 240.degree. 0.90
.mu.m 0.55 .mu.m -10.degree. 20.degree. 35.degree. -55.degree. X
left 2 .uparw. .uparw. .uparw. -80.degree. -40.degree. -35.degree.
55.degree. X 3 230.degree. 0.85 .mu.m 0.55 .mu.m -85.degree.
-40.degree. -35.degree. 40.degree. X left
__________________________________________________________________________
As can be seen from Table 1, when the angle between the direction
of the optical axis of the phase difference plate and the direction
of observation is between about 0.degree. and 30.degree., a display
is obtained which is easy to see and in which the contrast
distribution is generally symmetric with respect to the direction
of observation.
Examples 11 to 20 are display devices in which the axis of symmetry
is more perpendicular to the direction of observation. The results
of Examples 11 to 20, as well as Comparative Examples 4 to 6, are
shown in Table 2 below. The symbols .circleincircle.,
.largecircle., and x are used to indicate whether the contrast
curves are generally symmetrical with respect to a centerline
perpendicular to the direction of observation. .circleincircle.
indicates that the result is very good, .largecircle. indicates
good, and x indicates poor.
TABLE 2
__________________________________________________________________________
.DELTA.nf .multidot. df .DELTA.n .multidot. d of phase Twisting of
liquid difference Contrast Angle T crystal plate .THETA..sub.1
.THETA..sub.2 .THETA..sub.3 .THETA..sub.4 symmetry
__________________________________________________________________________
Examples of the invention 11 240.degree. 0.90 .mu.m 0.55 .mu.m
45.degree. 75.degree. 90.degree. 0.degree. .circleincircle. left 12
.uparw. .uparw. .uparw. 60.degree. 90.degree. -75.degree.
15.degree. .circleincircle. 13 .uparw. .uparw. .uparw. 75.degree.
15.degree. -60.degree. 30.degree. .largecircle. 14 .uparw. .uparw.
.uparw. 15.degree. 45.degree. 60.degree. -30.degree. .largecircle.
15 .uparw. 0.85 .mu.m 0.60 .mu.m 45.degree. 75.degree. 90.degree.
-10.degree. .circleincircle. 16 .uparw. .uparw. .uparw. 50.degree.
75.degree. -80.degree. 0.degree. .circleincircle. 17 .uparw. 0.80
.mu.m 0.55 .mu.m 45.degree. 75.degree. 90.degree. 0.degree.
.circleincircle. 18 180.degree. 0.85 .mu.m 0.55 .mu.m 45.degree.
-50.degree. 90.degree. 0.degree. .circleincircle. left 19
230.degree. 0.85 .mu.m 0.55 .mu.m 40.degree. 85.degree. 90.degree.
-15.degree. .circleincircle. left 20 .uparw. .uparw. .uparw.
20.degree. 75.degree. 70.degree. -35.degree. .largecircle.
Comparative examples 4 240.degree. 0.90 .mu.m 0.55 .mu.m 80.degree.
-20.degree. -55.degree. -35.degree. X left 5 .uparw. .uparw.
.uparw. 10.degree. 50.degree. 55.degree. -35.degree. X 6
230.degree. 0.85 .mu.m 0.55 .mu.m 5.degree. 40.degree. 55.degree.
-50.degree. X left
__________________________________________________________________________
As can be seen from Table 2, when the angle between the direction
of the optical axis of the phase difference plate and the direction
of observation is between about 60.degree. and 90.degree., a
display is obtained which is also easy to see and in which the
contrast distribution is generally symmetric with respect to a line
perpendicular to the direction of observation.
A liquid crystal display device 70 constructed and arranged in
accordance with a second embodiment of the invention is shown in
FIG. 7. Device 70 includes a twisted nematic liquid crystal cell 33
and two phase difference plates 72a and 72b. Phase difference plate
72a is adjacent liquid crystal cell 73 while phase difference plate
72b is adjacent upper linear polarizer 71. Cell 73 includes a
twisted nematic liquid crystal material 79 disposed between an
upper substrate 75 and lower substrate 76 with transparent
electrodes 77 and 78 disposed on the inner surfaces thereof. A
spacer 82 holds substrates 75 and 76 apart and retains liquid
crystal material 79 in the cell.
The optical relationships between the axes of polarizers 71 and 74,
phase difference plates 72a and 72b, and liquid crystal cell 73 are
shown in FIG. 8. The rubbing directions of upper substrate 75,
lower substrate 76, and phase difference plates 72 and 72b are
denoted by R75, R76, R72a and R72, respectively. The polarizing
axes (absorption axes) of linear polarizers 71 and 74 are denoted
by P71 and P74, respectively. Line A--A' represents the direction
of observation.
The direction and angle of twisting of liquid crystal material 79
in liquid crystal cell 73 as viewed in FIG. 7 from above to below
is designated as T. The angle between the observation direction
A--A' and direction P71 of the polarization axis of upper
polarizing plate 77 is designated as .THETA..sub.1. Similarly, the
angle between the observation direction A--A' and direction P74 of
the polarization axis of lower polarizing plate 74 is designated as
.THETA..sub.2. The angle between the observation direction A--A'
and the direction R72a of the optical axis of the phase difference
plate 72a is designated .THETA..sub.83a. Similarly, the angle
between the observation direction A--A' and the direction R72b of
the optical axis of the phase difference plate 72b is designated
.THETA..sub.83b. The angle between the observation direction A--A'
and the rubbing direction R75 of upper substrate 75 is designated
as .THETA..sub.4.
A liquid crystal display device was assembled having the
construction of device 70 with the following parameters:
1. liquid crystal material 79 has a counterclockwise twisting angle
of T=240.degree.;
2. .DELTA.n.multidot.d=0.90 .mu.m;
3. .DELTA.nf.multidot.df for phase difference plate 72a is 0.40
.mu.m;
4. .DELTA.nf.multidot.df for phase difference plate 72b is 0.45
.mu.m;
5. .THETA..sub.1 =-20.degree.;
6. .THETA..sub.2 =70.degree.;
7. .THETA..sub.83a =0.degree.;
8. .THETA..sub.83b =50.degree.; and
9. .THETA..sub.4 =-80.degree..
The parameters for Example 21 are set forth in Table 3 below, as
are the parameters for a further Example 22.
TABLE 3
__________________________________________________________________________
.DELTA.nf .multidot. df .DELTA.nf .multidot. df .DELTA.n .multidot.
d of phase of phase Twisting of liquid difference difference
Contrast Examples Angle T crystal plate 73a plate 73b .THETA..sub.1
.THETA..sub.2 .THETA..sub.83a .THETA..sub.83b .THETA..sub.4
symmetry
__________________________________________________________________________
21 240.degree. 0.90 .mu.m 0.40 .mu.m 0.45 .mu.m -20.degree.
70.degree. 0.degree. 50.degree. -80.degree. .circleincircle. left
22 240.degree. 0.90 .mu.m 0.42 .mu.m 0.42 .mu.m 0.degree.
90.degree. 30.degree. 70.degree. -70.degree. .largecircle. left
__________________________________________________________________________
The equal-contrast curves of Examples 21 and 22 are symmetric with
respect to the direction of the liquid crystal cell, and the
display is easy see under these conditions. However, the degree of
symmetry of equal-contrast curves is slightly lower in Example 22
than in Example 21.
A liquid crystal display device 90 constructed and arranged in
accordance with a third embodiment of the invention is shown in
FIG. 9. Device 90 includes twisted nematic liquid crystal cell 93
disposed between two phase difference plates 92a and 92b. Phase
difference plate 92a is between liquid crystal cell 93 and upper
linear polarizer 91 and phase difference plate 92b is between
liquid crystal cell 93 and lower linear polarizer 94. Cell 93
includes a twisted nematic liquid crystal material 99 disposed
between an upper substrate 95 and lower substrate 96 with
transparent electrodes 97 and 98 disposed on the inner surfaces
thereof. Spacer 102 holds substrates 95 and 96 apart and retains
liquid crystal material 99 therebetween.
The optical relationships of the axes of polarizers 91 and 94,
phase difference plates 92a and 92b, and cell 93 are shown in FIG.
10. The rubbing directions of upper substrate 95, lower substrate
96, and phase difference plates 92 and 92b are denoted by R95, R96,
R92a and R92, respectively. The polarizing axes (absorption axes)
of linear polarizers 91 and 94 are denoted by P91 and P94,
respectively. Line A--A' represents the direction of
observation.
The direction and angle of twisting of liquid crystal material 99
in liquid crystal cell 93 as viewed in FIG. 9 from above to below
is designated as T. .THETA..sub.1 is the angle between observation
direction A--A' and direction P91 of the polarization axis of upper
polarizing plate 91. Similarly, .THETA..sub.2 is the angle between
observation direction A--A' and direction P94 of the polarization
axis of lower polarizing plate 94. .THETA..sub.103a is the angle
between observation direction A--A' and the direction R92a of the
optical axis of the phase difference plate 92a. Similarly,
.THETA..sub.103b is the angle between observation direction A--A'
and direction R92b of the optical axis of phase difference plate
92b. .THETA..sub.4 is the angle between observation direction A--A'
and the rubbing direction R95 of upper substrate 95.
A liquid crystal display device having the construction of device
90 was assembled with the following parameters:
1. liquid crystal material 99 has a counterclockwise twisting angle
of T=240.degree.;
2. .DELTA.n.multidot.d=0.90 .mu.m;
3. .DELTA.nf.multidot.df of phase difference plate 92a is 0.42
.mu.m;
4. .DELTA.nf.multidot.df of phase difference plate 92b is 0.42
.mu.m;
5. .THETA..sub.1 =-10.degree.;
6. .THETA..sub.2 =-80.degree.;
7. .THETA..sub.103a =30.degree.;
8. .THETA..sub.103b =-30.degree.; and
9. .THETA..sub.4 =-60.degree..
The parameters defined above for Example 23 are set forth in Table
4 below, as are the parameters for a further Example 24.
TABLE 4
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.DELTA.nf .multidot. df .DELTA.nf .multidot. df .DELTA.n .multidot.
d of phase of phase Twisting of liquid difference difference
Contrast Examples Angle T crystal plate 103a plate 103b
.THETA..sub.1 .THETA..sub.2 .THETA..sub.103a .THETA..sub.103b
.THETA..sub.4 symmetry
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23 240.degree. 0.90 .mu.m 0.42 .mu.m 0.42 .mu.m -10.degree.
-80.degree. 30.degree. -30.degree. -60.degree. .circleincircle.
left 24 260.degree. 0.85 .mu.m 0.40 .mu.m 0.40 .mu.m 0.degree.
90.degree. 40.degree. -40.degree. -50.degree. .circleincircle. left
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The equal-contrast curves of Examples 21 and 22 are symmetric with
respect to the direction of the liquid crystal cell, and the
display is easily viewed under these conditions.
From the Examples, it can be seen that improved viewing angle
characteristics in the liquid crystal display device are obtained
in accordance with the invention when the direction of the optical
axis of the phase difference plate is angled to the observation
direction, preferably between about 0.degree.-30.degree. or
60.degree.-90.degree.. This is true whether one or two phase
difference plates are placed between the liquid crystal cell and
the upper polarizing plate, or if one phase difference plate is
placed between the liquid crystal cell and the upper polarizing
plate and another between the liquid crystal cell and the lower
polarizing plate. In this manner, a liquid crystal display device
is obtained which has improved viewing angle characteristics as
well as excellent black and white hues in the display.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently
attained and, since certain changes may be made in the above
article without departing from the spirit and scope of the
invention, it is intended that all matter contained in the above
description and shown in the accompanying drawings shall be
interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended
to cover all of the generic and specific features of the invention
herein described and all statements of the scope of the invention
which, as a matter of language, might be said to fall
therebetween.
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